Measurement of tidal volume in conscious, unrestrained mice has traditionally been performed using a whole-body plethysmograph (WBP). Such a device includes a chamber in which a mouse is placed. Pressure changes in the chamber due to respiration are observed, which are then related to tidal volume. The advantages of this type of system over a restrained plethysmograph are the extreme ease of use, reduced stress on the animal, and the ability for repeated and prolonged measurements.
Drorbaugh and Fenn related tidal volume to the pressure changes measured in a closed chamber due to thermo-hygrometric differences between respired air and gas within the chamber. (Drorbaugh, J. E. and W. O. Fenn. A barometric method for measuring ventilation in newborn infants. Pediatrics, 16:81-87, 1955.) Epstein and Epstein later pointed out a systematic error when only inspiratory events are used to calculate tidal volume. (Epstein, M. A. and R. A. Epstein. A theoretical analysis of the barometric method for measurements of tidal volume. Respir Physiol, 32:105-120, 1978.) Epstein et al., later proposed a method to account for these systematic errors. (Epstein, R. A., M. A. Epstein, G. G. Haddad, and R. B. Mellins. Practical implementation of the barometric method for measurement of tidal volume. J Appl Physiol, 49:1107-15, 1980.) At nearly the same time, Jacky also proposed an improved method of analysis that permitted long term measurements of tidal volume. (Jacky, J. P. Barometric measurement of tidal volumes: effects of pattern and nasal temperature. J Appl Physiol, 49:319-325, 1980.) All barometric plethysmograph techniques assume that changes in plethysmograph pressure can be accounted for solely by changes in temperature and humidity.
It is known, however, that gas compression in the lung can also contribute significantly to the pressure measured in an unrestrained plethysmograph. Frazer et al., demonstrated the effects of compression on the WBP signal with a model validated by simultaneously measuring plethysmograph pressure and chest wall motion of guinea pigs with a laser displacement sensor. (Frazer et al. Estimation of guinea pig airway resistance following exposure to cotton dust measured with a whole body plethysmograph. In: Proceedings of the Twenty-First Cotton and Organic Dust Research Conference, edited by R. R. Jacobs and P. J. Wakelyn, vol. 12, pp. 171-174. 1997.) Enhorning et al., used a mechanical model of the chest to show that plethysmographic pressure was not only affected by breathing pattern, but also by airway resistance. (Enhorning et al. Whole-body plethymography, does it measure tidal volume in small animals? Can J Physiol Pharmacol, 76:945-951, 1989.) Although there is some controversy as to the extent and conditions under which gas compression becomes a significant portion of the WBP signal measured in mice, it is clear that tidal volume measurements of mice with increased airway resistance or breathing rate are likely to contain a significant component related to gas compression.
Further, previous attempts to measure specific airway resistance with a conventional WBP have failed largely because changes in these measurements can be explained not only by changes in airway resistance, but also by changes in the tidal volume breathing pattern. Lundblad et al., showed that gas compression could be estimated by conditioning the chamber air to near alveolar conditions, but that estimates of airway resistance still require knowledge of the tidal breathing pattern. (Lundblad, L. K. A., C. G. Irvin, A. Adler, and J. H. T. Bates. A reevaluation of the validity of unrestrained plethysmography in mice. J Appl Physiol, 93:1198-1207, 2002.)